A Scotland Yard anti-corruption officer was arrested on Wednesday on suspicion of taking bribes from private investigators.
Two private investigators - who both previously served with the force - are also being held after raids at the London offices of Risc Management Ltd, it is understood.
The Met said in a statement: "Officers from the Directorate of Professional Standards executed a search warrant at a business address in Conduit Street, London this morning and arrested two men."
The 45-year-old detective constable is being questioned on suspicion of providing information about Nigerian fraudster James Ibori in return for cash.
Mike Schwarz, a partner at Bindmans LLP, told the Home Affairs Select Committee yesterday of possible cash payments made by investigation firm Risc to sources of theirs who were "presumably police officers or those close to the investigation".
Records showed roughly half a dozen payments amounting to about £20,000 over an eight or nine-month period, Mr Schwarz claims.
Scotland Yard said the fourth man was arrested at a south London police station by appointment.
"He was arrested on suspicion of bribery of a police officer," a spokesman said.
"Officers have previously attended a residential address outside of London and a 45-year-old man was arrested on suspicion of receiving payments for information. He is a serving detective constable in the MPS (Metropolitan Police Service)."
The two private detectives were arrested at Risc's business address as part of a "pre-planned operation that forms part of a long and complex investigation by the directorate of professional standards (DPS)".
A statement from the force added: "The Metropolitan Police Service is investigating an allegation that illegal payments were made to police officers for information by a private investigation agency.
"The DPS referred the matter to the Independent Police Complaints Commission in October 2011 which agreed to supervise a DPS investigation into the allegations.
"Officers from the DPS have today executed search warrants at a business address in Conduit Street."
Risc was hired by the law firm representing Ibori, a former governor of one of Nigeria's richest oil-producing states who was brought to justice in London.
He was jailed for 13 years last month after admitting fraud totalling nearly £50 million.
Mr Schwarz, whose firm represented one of Ibori's co-accused, told MPs: "What I have seen in that case is serious illegality on behalf of private investigators Risc Management Ltd ... which involves apparent corruption right at the heart of New Scotland Yard.
"The core as I see it is the operation of the police and their connection with the security firm involved, the police being the proceeds of corruption unit within New Scotland Yard.
"And the problem there is the key culprits appear to be the key players, who are the senior investigating police officer ... and two of the key investigators."
Mr Schwarz told MPs he had seen material submitted to the committee involving invoices from Risc Management to Ibori's solicitors reporting "payments made by Risc Management to sources that they have, presumably police officers or those close to the investigation".
He added: "It appears to be inappropriate, if not corrupt."
Ibori pleaded guilty at Southwark Crown Court in February to a series of charges linked to the theft of money from the Delta state and fraud involving state-owned shares in a mobile phone firm.
The Nigerian, whose address in England was given as Primrose Hill in north London, was previously a cashier in a DIY store in Ruislip, west London, before he moved to Nigeria and worked his way up the ranks to become a state governor in 1999.
He reigned as governor until 2007 and was extradited from Dubai to the UK last April.
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